Selective breeding for high and low levels of opiate-induced analgesia in mice

Beginning with a genetically heterogeneous outbred stock of mice (Binghamton HET), selective breeding was conducted within two selected lines for a high analgesic (antinociceptive) response and a low analgesic response, respectively, to a narcotic analgesic (levorphanol tartrate) using the hot-plate test. A nonselected control line was also maintained concurrently. Four generations of predominantly mass selection have produced a marked divergence between the two oppositely selected lines, yielding a realized heritability (h super(2)) of 0.32 plus or minus 0.05. The selection response was markedly asymmetrical, with a realized h super(2) in the high direction of 0.45 plus or minus 0.10 and in the low direction of 0.12 plus or minus 0.07.